1. Field of the Invention
The present invention generally relates to signal processing circuits and methods, and more particularly to a signal processing circuit and method for extracting a signal superimposed on a reference signal varying in level at a given frequency, the superimposed signal being inserted into the reference signal at given positions of a given level thereof.
2. Description of the Related Art
Recordable-type disks having grooves for information recording and reproduction, such as CDs and DVDs, include those conventionally known disks that have grooves meandering radially to form a wobble. Also known as such recordable-type disks are those having pre-pits formed on lands between adjacent grooves so as to synchronize intermittently with a wobble formed by the grooves. Hereinafter, such pre-pits are referred to as land pre-pits (LPPs).
A disk unit for recording information on and reproducing information from such disks includes an optical head opposing the surface of a disk attached to the disk unit. The optical head records information on the disk by emitting a laser light thereonto, and outputs a reproduced signal corresponding to the information recorded on the disk by receiving a reflected light from the disk. The information reproduced by the optical head includes a signal deriving from a wobble formed on the disk (hereinafter, referred to as a wobble signal) and a signal deriving from LPPs formed on the disk (hereinafter, referred to as an LPP signal). The disk unit extracts the wobble signal and the LPP signal from the information reproduced by the optical head. Then, based on the extracted signals, the disk unit obtains address information indicating the position of the disk, and controls the spindle motor rotating the disk. Accordingly, in order to have the disk unit function properly, it is necessary to generate, from the LPPs formed on the disk, the LPP signal complying properly with the LPPs.
In the case of a DVD-R/RW disk, the LPPs are formed as address information since no address information is superimposed on the wobble signal.
The LPPs are formed to have a given phase relation to the wobble signal. Normally, the LPPs are formed so that the LPP signal is superimposed on the wobble signal serving as a baseline voltage at the positions of the maximum amplitude of the wobble signal.
The DVD-R/RW disk is defined so as to include eight wobble waves in one EFM sync frame, and the address of the DVD-R/RW disk is represented by three-bit data expressed by the presence (=1) or absence (=0) of an LPP in the first three waves.
As described above, the LPPs formed on the disk synchronize intermittently with the wobble. Therefore, if the LPPs are detected based on the LPP signal only during a period in which the wobble signal is set to HIGH (at a high level), or at a given threshold value or over, noise can be prevented from being mistakenly detected as the LPP signal, so that the accuracy of LPP detection can be improved.
However, if the LPPs are detected based on the LPP signal only during the period in which the wobble signal generated based on the wobble of the disk is set to HIGH, the LPPs are detected even during a period in which noise is superimposed on the wobble signal due to dust or flaws on the surface of the disk. As a result, the accuracy of LPP detection is decreased. Accordingly, it is not adequate to base LPP detection timing only on the period in which the wobble signal is set to HIGH.